Using the same logic as the single Export/Import commands, implement the
entire backup command by exporting all the serializable classes into a
single json file.
To select the correct class upon import, save a new property on the
backup json, the class_, which contains the name of the class to be
imported.
Fix the note serialization.
The new command ImportCommand takes a Serializable type, from which it
can call the deserialize method to generate a DB ORM type. The
Serializable interface also declares the serialize method.
(De)serialization moved to the ORM types, due to the inability to
properly use overloading.
Possible improvement for the future is to merge serialization
information on JSONDecoder/Encoder classes.
Adds a MockClient with the in-memory SQLite DB which can be used by
tests.
Most types export/import functionally tested using two DBs and comparing
entries.
The `CategorySelector` was possibly added to be incremented with other
attributes. However, since none other that the selector enum is used at
the moment, it is only adding unnecessary cluter.
The category selector value is moved to the parent
`TransactionCategory`.
